A sight-reader is a person who can play or sing music from the notes the first time they look at them. They have not practised that piece before. Music students work on this because in a group, or in a test, new music arrives and you have to start. Though actually, even a good sight-reader will miss a note; they just do not stop, so the music still feels like music. You use this word for the person. The doing is called sight-reading. You will hear it in music class and among players, not much in the street.